1. Field of the Invention
The present invention relates to equipment for chemical processes. In particular, the present invention relates to equipment for growing a thin film in a reaction chamber.
2. Description of the Related Art
There are several vapor deposition methods for depositing thin films on the surface of substrates. These methods include vacuum evaporation deposition, Molecular Beam Epitaxy (MBE), different variants of Chemical Vapor Deposition (CVD) (including low-pressure and organometallic CVD and plasma-enhanced CVD), and Atomic Layer Epitaxy (ALE), which is more recently referred to as Atomic Layer Deposition (ALD).
ALD is a known process in the semiconductor industry for forming thin films of materials on substrates such as silicon wafers. ALD is a type of vapor deposition wherein a film is built up through self-saturating reactions performed in cycles. The thickness of the film is determined by the number of cycles performed. In an ALD process, gaseous precursors are supplied, alternatingly and repeatedly, to the substrate or wafer to form a thin film of material on the wafer. One reactant adsorbs in a self-limiting process on the wafer. A subsequent reactant pulse reacts with the adsorbed material to form a single molecular layer of the desired material. Decomposition may occur through reaction with an appropriately selected reagent, such as in a ligand exchange or a gettering reaction. In a typical ALD reaction, no more than a molecular monolayer forms per cycle. Thicker films are produced through repeated growth cycles until the target thickness is achieved.
In an ALD process, one or more substrates with at least one surface to be coated and reactants for forming a desired product are introduced into the reactor or deposition chamber. The one or more substrates are typically placed on a wafer support or susceptor. The wafer support is located inside a chamber defined within the reactor. The wafer is heated to a desired temperature above the condensation temperatures of the reactant gases and below the thermal decomposition temperatures of the reactant gases.
A characteristic feature of ALD is that each reactant is delivered to the substrate in a pulse until a saturated surface condition is reached. As noted above, one reactant typically adsorbs on the substrate surface and a second reactant subsequently reacts with the adsorbed species. As the growth rate is self-limiting, the rate of growth is proportional to the repetition rate of the reaction sequences, rather than to the temperature or flux of reactant as in CVD.
To obtain self-limiting growth, vapor phase reactants are kept separated by purge or other removal steps between sequential reactant pulses. Since growth of the desired material does not occur during the purge step, it can be advantageous to limit the duration of the purge step. A shorter duration purge step can increase the available time for adsorption and reaction of the reactants within the reactor, but because the reactants are often mutually reactive, mixing of the vapor phase reactants should be avoided to reduce the risk of CVD reactions destroying the self-limiting nature of the deposition. Even mixing on shared lines immediately upstream or downstream of the reaction chamber can contaminate the process through parasitic CVD and subsequent particulate generation.